Armor Class is really fucking stupid.
So I put on platemail, and I become harder to hit? Dummy. I know guys in heavy armor were surprisingly agile and fast, but putting on thick steel over gambeson just slows you down. It is great at stopping blows that do land from making you bleed and die, however.
So if armor is damage reduction, how do we hit someone? Melee combat is an exchange of blows*. Much like boxing, your looking for a series of telling blows that will earn you points. Or corpses. If we want to simulate that exchange both sides needs to roll. Static numbers won't do, because we need the winds of fate. So however you work out an attack roll, mirror it, then you've got a defence roll. A chance for the attack to land a blow, and the defender to turn it. Ever have players saying "Aww can't I parry it?" Not a problem anymore.
I changed the d20 mechanic to an Arnesonian roll–below–attribute system, so its sweet simplicity mirrored for a defender to roll below an attribute also. We can the parse the outcome and do something interesting with them. Win-Win. Win-Lose. Lose-Win. Lose-Lose. Plus there's crits with each roll (currently natural 1, no confirming)
Once you start doing all this, then you're looking a re-write of all the core systems you'd expect from a Medieval Fantasy RPG - not now mind you, another time.
*Not that i'd actually know, Im only lightly familiar with re-inactment. The closest I can get is watching the full contact melee's on youtube - and even these are blunt weapons with no kill strikes. On the other hand i'd remind myself that it also needs to be gameable and fun so essentially, fuck it.